Figure 36–1 The center of mass is controlled by mov-
ing the center of pressure.
A.The force caused by gravity passes through the center
of mass (CoM) in the trunk. The surface exerts an upward
force against each foot, such that the ground reaction
force vector originates at the center of pressure (CoP) on
the support surface. Below: Even when the feet remain
in place, the CoP (blue displacement) and CoM (gold
displacement) are always in motion as we sway. During
normal standing on two feet, the projection of the CoM of
the body remains within the base of support (light blue
rectangle around the feet in contact with the ground) for
equilibrium. The base of support of the standing human is
defined by contact of the feet on the support surface.
B.In a dynamic situation, such as standing on one leg on a
narrow beam, equilibrium can be maintained even when the
body CoM displacements (gold displacements) go outside of
the base of support for brief periods. Strategies such as
counter-rotation of the lower and upper body can tilt the ground
reaction force so that it accelerates the body CoM back over its
base of support. (Adapted, with permission, from Otten 1999.
Permission conveyed through Copyright Clearance Center, Inc.)
The center of pressure (CoP) is defined as the origin
of the ground reaction force vector on the support sur-
face. For the body to be in static equilibrium, that is,
to remain motionless, the force caused by gravity and
the ground reaction force must be equal and opposite,
and the CoP must be directly under the center of mass
(CoM) (Figure 36–1A). Misalignment of the CoM and
CoP causes motion of the CoM. For example, if the
CoM projection onto the base of support is to the right
of the CoP, the body will sway to the right until the CoP
moves to the right to move the CoM back over the base
of support.
However, standing is never truly static. While the
body is in motion (postural sway), CoM and CoP are not
aligned and dynamic equilibrium must maintain bal-
ance (Figure 36–1B). In fact, when the body is unsup-
ported, CoP and CoM are continually in motion and are
rarely aligned, although when averaged over time dur-
ing quiet stance, they are coincident. The sway of the
body during quiet stance can be described by the trajec-
tory of either CoM or CoP over time, such as sway path,
area, velocity, and frequency.
In more dynamic situations like walking, running,
turning, and jumping, stability can be achieved even
when the CoM briefly goes outside the base of support.
For example, when standing on one leg or on a narrow
beam, momentum from rotating the hips, arms, and
other body parts or movement of the CoP can be used
to change the direction of the ground reaction force to
return the body CoM over its base of support to main-
tain stability (Figure 36–1B). If the CoM is outside the
base of support and heading away from it, subjects may
need to take a step or grab a stable object to change the
base of support and avoid a fall.
Box 36–1 Center of Pressure
测力板
支撑面
B.